Project: Application of a novel geochemical approach to the alkalinity anomaly method of estimating coral reef calcification rates: implications of ocean acidification

With the recent awareness that the surface oceans are becoming more acidic due to the uptake of anthropogenic CO2, and that the resulting decrease in the carbonate mineral saturation state causes a decline in the calcification rate of many organisms, including corals, there is a pressing need to establish baseline calcification rates of coral reefs against which future changes can be measured. The standard method for measuring coral reef calcification requires knowledge of the alkalinity depletion relative to the offshore source water and the residence time of the water over the reef. To compute the residence time it is generally necessary to measure or model the flux of water into and out of the system. This can require a great deal of time and money and greatly limits the utility of this otherwise powerful method.

In this project, two researchers at the University of Miami -- a coral ecologist and a radioisotope geochemist -- will use a novel radiochemical method based on the cosmogenic radioisotope 7Be to estimate the residence from a simple set of inexpensive measurements. The isotope 7Be has an atmospheric input to the ocean via rainfall. Rainfall events act as natural "tracer release" experiments, whereby in shallow coral reef waters this flux results in a highly concentrated 7Be activity, while in offshore waters the same flux is distributed over a much deeper mixed layer resulting in a much lower 7Be activity. With knowledge of the input flux (via precipitation), the persistence of this activity contrast can be used to establish the residence time of water over the reef over a timescale of 1-20 days. This is the range within which most reef systems should fall. The team will employ the method to determine how calcification rate varies spatially and temporally at three well-studied reef systems in the Caribbean and Western Atlantic.

Broader impacts: Coral reefs are one of the ecosystems most likely to show the first signs of stress in response to global warming and ocean acidification. Lab studies have shown that coral calcification declines linearly with the decline in saturation state that is resulting from the increase in atmospheric CO2. However, lab studies have also shown that the response can be non-linear when combined with rising temperature. There is a real possibility that calcification on coral reefs will fall below the threshold rate needed to sustain themselves against normal erosive forces in the next 50-100 years. In order to ascertain the impact of climate change new methods are needed to extend the spatial and temporal coverage of coral reef studies so that it can be judged whether the changes are global in scope and temporally coherent with observed changes in climate and ocean chemistry. This project features the development, application, and evaluation of a method that would advance the synopticity of reef metabolic studies. As an aside, this is a potential tool for evaluating flushing of pollution events over reef systems and could be beneficial in remediation efforts.